The invention relates generally to the field of variable geometry guide vanes for gas turbine engines. More specifically, the invention relates to variable geometry guide vane assemblies that reduce stress placed on downstream compressor blades.
A gas turbine engine compressor typically includes inlet guide vanes followed by a row, or stage of compressor rotor blades. A fan (military style) or high pressure compressor will only have one row of inlet guide vanes. There may be other rows of variable vanes, but they may differ in their principle of operation. During operation, air is sequentially compressed by the compressor stages. The compressed air is channeled to a combustor and mixed with fuel and ignited. The hot combustion gases generated power the engine.
Axial compressors rely on spinning blades that have airfoil sections similar to airplane wings. As with airplane wings, in some conditions the blades can stall or surge. If this occurs, the airflow around the stalled compressor can reverse direction violently. Many compressors are fitted with anti-stall systems such as bleed bands or variable geometry guide vanes to decrease the likelihood of surge.
To ensure compressor stability over a wide range of mass flow rates and operating speeds, variable guide vanes are employed. Guide vanes are usually cast structures having an airfoil and a platform. The aerodynamic vanes turn the airstreams through an angle to meet the blades of a following compressor stage and reduce the effective inlet area of the stage.
Variable guide vane assemblies use blades that can be individually rotated around their axis, as opposed to the power axis of the engine. For startup they are rotated to open, reducing compression, and then are rotated back into the airflow as operating conditions require. Closing the guide vanes progressively as compressor speed falls reduces the slope of the surge (or stall) line, improving the surge margin of the engine.
Vane movement is accomplished by coupling a corresponding vane arm to the outer ends of each vane and joining the vane arms to a common actuation or unison ring for providing uniform adjustment of the individual vanes. Each vane must be identically angled relative to the other vanes in the ring to maximize efficiency and prevent undesirable aerodynamic distortion from a misaligned vane.
Current variable geometry inlet guide vanes are positioned radially around the longitudinal engine axis. A typical variable inlet guide vane assembly is shown in FIG. 1. A problem experienced with current variable geometry guide vane designs is a stress that manifests itself at the root, or inner radial ends of the downstream compressor blades. The high stress experienced is due to unsteady air formed at their outer radial ends. The unsteady air pushes and pulls on the blades, stressing where they couple to an inner concentric engine structure.
Radial inlet guide vanes do not direct a uniform velocity of air across the downstream compressor blades as their geometry changes in response to engine demands. As a result, the compressor blades experience an unbalanced loading of air velocities with slower moving, separated air concentrated near the outer radial end regions.
What is desired is a variable geometry guide vane assembly that reduces unwanted compressor blade or fan blade stresses. The invention provides a solution to this problem.